14-08-2011 12:27

=Martin=Taylor=

Re: pic: Don't do this. It's a bad idea.

Not a suspension.

What's going on in those wheels? Looks like a lot of linkages.

14-08-2011 12:44

Cuog

Re: pic: Don't do this. It's a bad idea.

Looks like a psuedo suspension. The wheels look to be able to tilt up/down which would help a little in covering uneven terrain. One problem is that the pivot for the suspension is in a different plane than the U-Joint so there would be a tendency to try and stretch the U Joint unless it has something like a slip-yoke.

14-08-2011 12:44

Marc S.

Re: pic: Don't do this. It's a bad idea.

Looks like a possible crab module, with 2 wheels spread apart for stability, a differential in the center for better turning and U-joints for smooth suspension. Maybe next years surface will be very bumpy.

14-08-2011 12:49

MrForbes

Re: pic: Don't do this. It's a bad idea.

Looks like the robot version of an automotive Independent Rear Suspension (IRS).

That's a really neat project to do when you're bored! nice

14-08-2011 13:02

SenorZ

Re: pic: Don't do this. It's a bad idea.

I think a rough terrain game is inevitable... and would be very cool. There seems to be a lot of teams that perfect their preferred drive train in the off season and spend the time during build season focusing on game play. Making a bumpy playing field could help, er, level the playing field for newer teams... like mine!

14-08-2011 13:28

Chris is me

Re: pic: Don't do this. It's a bad idea.

Okay, I'm going to try and describe this. It's a swerve module with a differential connecting both wheels, and flex couplings to allow for some sort of suspension.

I can't possibly imagine its use in FRC.

14-08-2011 13:38

Andrew Schreiber

Re: pic: Don't do this. It's a bad idea.

Coaxial swerve modules with a suspension and differential aren't normal. But on Winnovation they are.

14-08-2011 14:01

Michael Blake

Re: pic: Don't do this. It's a bad idea.

I can't figure out why you have four Bevel gears (forming a box, differential?) connected to the left wheel... couldn't you achieve the same by extending the shaft from the right wheel directly to drive the left wheel?

THANKS!

14-08-2011 14:05

lemiant

Re: pic: Don't do this. It's a bad idea.

Actually the box results in the two turning in opposite directions.

14-08-2011 14:08

MrForbes

Re: pic: Don't do this. It's a bad idea.

The differential is used in the drive axle of cars, read about it

http://en.wikipedia.org/wiki/Differe...ical_device%29

14-08-2011 14:14

Michael Blake

Re: pic: Don't do this. It's a bad idea.

RIGHT, but you wouldn't want them to turn oppositely, correct?

This has me scratching-my-head... ;-)

14-08-2011 14:17

EricH

Re: pic: Don't do this. It's a bad idea.

Actually, Michael, lemiant didn't have a chance to read squirrel's link yet. I would suggest reading that link, then figuring out the drive system.

Short version: If both can go the same way at the same speed, both go the same way at the same speed. But if one of them can't keep that speed up, then it doesn't have to due to the way the differential is set up.

14-08-2011 14:19

Tristan Lall

Re: pic: Don't do this. It's a bad idea.

Wouldn't this work better with constant velocity joints (rather than universal joints)?

And are you using the universal joint as two of the members in a four-bar linkage that holds up the wheel? If so, isn't it just going to deflect to the maximum? (Or am I missing a spring somewhere?)

This would be a fun stock design to keep around...just to tempt people when FIRST decides to give us some real obstacles.

14-08-2011 14:21

Cuog

Re: pic: Don't do this. It's a bad idea.

http://www.youtube.com/watch?v=vBm-S...eature=related

Here's an animation of how a differential operates for those confused.

14-08-2011 14:35

MrForbes

Re: pic: Don't do this. It's a bad idea.

Here's a much better explanation.....but then, I do like old stuff

http://www.youtube.com/watch?v=F40ZBDAG8-o

14-08-2011 14:35

Michael Blake

Re: pic: Don't do this. It's a bad idea.

So, where's the "slippage" provided in this setup?

THANKS!

14-08-2011 14:41

Hawiian Cadder

Re: pic: Don't do this. It's a bad idea.

Why not choose pneumatic tires over the suspension? I don think you would get much "play" before the bevel gear bottoms out.

14-08-2011 14:43

Cuog

Re: pic: Don't do this. It's a bad idea.

There is no "slip"* in a differential. The nature of how the gears mesh is what allows the shafts to turn at differing rates. The differential itself has no control over the situation, its just along for the ride.

*(unless we start talking limited slip differentials and lockers etc)

14-08-2011 14:45

EricH

Re: pic: Don't do this. It's a bad idea.

In the differential gearing. Two of the 4 gears in the inner box are connected to the wheels; the other two are idlers. The relative speed of the wheels doesn't really matter to the idlers; they'll move as fast as they need to.

Hint: Think of this setup as 2 gearboxes. The inner box is the 4 in the middle of the "axle"; the outer is the bevel at the top and the large gear it meshes with.

14-08-2011 14:45

Michael Blake

Re: pic: Don't do this. It's a bad idea.

NOW, _that_ was helpful... ;-) THANKS, Jim!!

14-08-2011 14:56

Michael Blake

Re: pic: Don't do this. It's a bad idea.

Also, helpful... THANKS, Eric!

14-08-2011 15:01

lemiant

Re: pic: Don't do this. It's a bad idea.

I hadn't. I thought the big bevel was attached to the shaft not the box. After reading that article I understand, and am amazed!!!!

14-08-2011 15:04

AdamHeard

Re: pic: Don't do this. It's a bad idea.

I Agree with your complaints about suspension geometry.

Also, The suspension on this seems to indicate a purpose of driving over varied terrain. With the completely open differential on there, the wheel with less resistance (often in the air, not making any contact) will get all of the rotation, leaving a torque on the static wheel equivalent to just the friction in the differential.

14-08-2011 15:22

Hawiian Cadder

Re: pic: Don't do this. It's a bad idea.

true, maybe you could use a limited slip differential to prevent that.

14-08-2011 16:13

PAR_WIG1350

Re: pic: Don't do this. It's a bad idea.

The suspension Appears to actually a three bar linkage with the link from the wheel bearing retainer to the differential frame being a spring loaded variable length link (possibly a gas shock). The u-joint is simply the joint between two links. It would benefit from more support, but the geometry should still work.

Also, by linking the axles with a slip clutch between the output gears of the differential the static wheel issue could be resolved. Like this. basically it's a primitive version of the limited slip differential.

14-08-2011 16:58

MrForbes

Re: pic: Don't do this. It's a bad idea.

That method of using the axle shaft as a suspension locating member was used on some IRS cars for years, including the Corvette starting in 1963

14-08-2011 17:02

Cuog

Re: pic: Don't do this. It's a bad idea.

Also known as a "Swing axle"

First on this list: http://www.autozine.org/technical_sc...uspension2.htm

Also used on VW Beetles(busses, and most other models including porsche) 38-68

14-08-2011 18:28

MrForbes

Re: pic: Don't do this. It's a bad idea.

There are different ways of doing it, the Corvette used Ujoints at both ends of the axle shaft, so it isn't really a swing axle like the early VW. The axle and control arm make a parallel arm setup, so the camber doesn't change dramatically like on the early bug

14-08-2011 23:25

Dillon Carey

Re: pic: Don't do this. It's a bad idea.

This was just a fun project, we probably will never make anything like this.

Also I thought about making it into a limited slip diff. but I ran out of steam before getting that far.

As far as the suspension is concerned, I put it in there just so if the robot got tipped a little by another bot, it could still get some power to the floor. And I have never done anything with any sort of suspension, so I'm sure it is probably a terrible setup.

Last, the entire reasoning behind a differential module was to be able to have lots of traction, while still being able to turn the module.

15-08-2011 01:11

Borisdamole

Re: pic: Don't do this. It's a bad idea.

but Corvette is using that strange leaf spring setup....

And this drive train could improve traction by using a sealed diff, and using silicone diff fluid to prevent the module from "diffing out"

A limited slip diff is normally all mechanical, and uses helical gears to prevent "diffing out" (but there are others that operate on clutches or springs)

a traditional limited slip setup would be VERY costly for this size (the gears would be expensive), but silicone would be a great option....

Many small differentials are easily available from any rc car shop or ebay

Cool idea!

15-08-2011 01:31

Aren Siekmeier

Re: pic: Don't do this. It's a bad idea.

Odd how this comes out the day before I return from a week long vacation when I had enough time to think this up in my head. This is exactly what I was envisioning, mechanically anyways: a swerve module that can be pivoted effortlessly due to the differential box.

Several people have explained it already, but I'll take a stab too. You turn the coax shaft as usual and it turns the differential box turning both wheels, like a normal differential drive (this also allows for some interesting turning cases without most of the wheel scrub). Turning the module drives the wheels in opposite directions while the differential box remains stationary. He added the fourth bevel in there just cuz (for strength?), it seems to me, as most differentials operate with only three. Now try flipping one wheel's input to the box (by putting the two perpendicular bevels on the same shaft or any of a number of other things), and now it can translate freely, but must be powered to rotate.... An interesting, though usually useless, prospect.

The universal joint suspension was definitely not something that entered my head. Why you would ever need that, no one knows. Then again, why would you ever need any of this? It's just awesome, that's all.

15-08-2011 09:08

Borisdamole

Re: pic: Don't do this. It's a bad idea.

Mechanically 3 will work, but it creates alot of load on the center gear.

Acutually many diffs use 4 center gears(total of 6 gears)
like this

15-08-2011 09:39

JesseK

Re: pic: Don't do this. It's a bad idea.

iirc, this is eerily similar to the front landing gear for planes.

15-08-2011 11:13

MrForbes

Re: pic: Don't do this. It's a bad idea.

Most of the automotive differentials I've seen use two "spider" gears, for a total of 4 gears. The only common exception is the heavy duty Ford 9" rear from the 1970s-80s. But I haven't worked on many late models either!

15-08-2011 11:30

Chris is me

Re: pic: Don't do this. It's a bad idea.

The suspension is definitely similar, but I thought the landing gear for planes were unpowered casters? I'm no expert by any means, though.

15-08-2011 11:58

Ian Curtis

Re: pic: Don't do this. It's a bad idea.

No, the nose gear is actuated for taxiing. The rudder is very ineffective at low speeds. Think about it, would flapping the rudder around give you a whole lot of moment when you are sitting on the ramp? Some very old planes have a tail skid in place of a nose wheel, and occasionally this isn't actuated (but it makes ground handling a royal pain).

15-08-2011 18:27

Garret

Re: pic: Don't do this. It's a bad idea.

I may be wrong, but I don't think that this will even offer the right type of suspension. From the picture it looks like there is only one universal joint on each side, meaning that the wheel can only rotate up and down, not just move up straight up and down as in a more traditional suspension. I would think that there would need to be at least 2 universal joints on each side in order to allow it to move up and down. However I do like the idea a lot.

15-08-2011 20:59

Tristan Lall

Re: pic: Don't do this. It's a bad idea.

On most aircraft using tricycle gear, the nose wheel is unpowered, unbraked and mechanically or hydraulically steered.

There are numerous exceptions, however. Some early Boeing 727s had nose gear brakes, but they were omitted on later aircraft (and often removed on early ones) because the added braking capacity was of little use compared to the weight penalty (something like a hundred pounds). There was an aftermarket kit being evaluated by Air Canada on one of their Boeing 767s which added an electric motor to the nosewheel for taxiing. While saving wear and tear on the main engines was a good idea (I believe it could run on the APU's generator), again, it weighed too much, and there was concern it could lead to higher maintenance costs for the nose gear assembly. On a few small aircraft (Diamond's DA20, for example), the nose wheel is freely castering. Steering is accomplished with the toe brakes controlling the main wheels (differential braking), and the rudder pedals at higher speeds.

Incidentally, the suspension on the nosewheel of most aircraft is not located beside the wheel like that—it's usually in the strut. (Gas-over-oil is common.)

16-08-2011 01:25

PAR_WIG1350

Re: pic: Don't do this. It's a bad idea.

Another exception would be the Boeing B-52 Stratofortress Which has a quadracycle undercarriage and 4-'module' crab steering to facilitate landing the behemoth of an aircraft in high crosswinds conditions. Most aircraft have to approach flying sideways and straighten out after they land to roll down the runway instead of off it, the B-52 simply rolls sideways .

16-08-2011 03:24

Tristan Lall

Re: pic: Don't do this. It's a bad idea.

To be fair, the B-52 only has ±20° of strafing ability.

Actually, the reason for those steerable gear is the use of ridiculously-small control surfaces on the B-52. The engineers designing it in the late 1940s were not confident in the reliability of early hydraulic flight controls, and therefore adopted all sorts of unusual compromises. Instead of using a large hydraulic rudder to control the plane in a crosswind, they just added steerable gear, and stuck with a very narrow (though still about 20 ft tall) spring-tab-assisted rudder.

It seems to have worked, because the early-1960s-built B-52Hs are likely to be in service for at least 30 more years....